
THE HISPANIC FOUNDATION 



Book 


FROM THE 

ARCHER M. HUNTINGTON 
PURCHASING FUND 




/ 


/ 




M c CALL U M’S 



EXPLAINED AND ILLUSTRATED. 






BY D;“'C:" MeCALLDM. 




Jjitt furls: 


SAMUEL T. CALLAHAN, STEAM PRINTER, 73 FULTON ST. 


1859. 























By Transfer 

NOV 21 1916 


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The following description of what is known as the 
“ McCallum Inflexible Arched Truss Bridge,” is 


given at the solicitation of a number of gentlemen of the 


engineering profession, to whom the writer has had the 
honor of explaining it verbally, with the aid of illustrative 
models and drawings, and has been deferred until this time 
in consequence of other engagements, and also, that suf¬ 
ficient time might elapse to test practically, and prove be¬ 


yond a reasonable doubt, the correctness of the theory 


involved in its combination and arrangement. 

In attempting to explain this peculiar form of truss, the 
writer would, at the outset, disclaim all desire to under¬ 
value or disparage the merits of any other plan of bridge in 
use, as each has its advocates, and all possess more or less 
merit. Inasmuch as no arrangement can be absolutely 
perfect, nothing more is claimed for the “ Inflexible Arched 
Truss,” than that its combination approximates nearer to 
the standard of a perfect structure, than any other plan of 
which the writer has any knowledge. 













4 


This much by way of introduction, and the remark may 
be ventured, that the most experienced in bridge construc¬ 
tion have yet much to learn, and it is but reasonable to 
conclude that, while the arts and sciences generally are 
making rapid strides in the path of progress, this particular 
branch can form no exception. The history of the past is 
fraught with lessons of significant import, teaching us in 
this, as in all other investigations, that modesty is a becom¬ 
ing virtue,—that however perfect in our estimation the 
present development of science may be, the march must 
still be onward, and the real question is, not whether we 
shall advance another step, but rather, what that step shall 
be. 

As this is not intended to be a treatise on bridge con¬ 
struction, but a narrative of facts connected with the 
writer’s experience on the subject, it will not be expected 
that more than the nature of the forces involved will be 
given, leaving the measurement and intensity of those forces 
to the student, who may be disposed to pursue a more 
elaborate investigation of the problems, requisite to a full 
understanding of this important branch of the profession. 

In order to place more clearly before the reader a state¬ 
ment of facts and experiments which led to the adoption of 
the “ Inflexible Arched Truss,” reference to other and 
well-known bridge structures will be indispensable, and 
they will be alluded to, only so far as may be essential to 
a proper understanding of the subject. 

No attempt will be made to discuss the merits or demer¬ 
its of the great number of plans of bridge structures now 
before the public (several of which have been used to a 
limited extent, while others exist in theory only), the 
claims of all of which are generally based upon some unim¬ 
portant alteration in detail, which in most case , resolves 














5 


» 

j 

itself into a distinction, without any real difference in mode 
of action. 

There are now in use miles in length of the “ Inflexible 
Arched Truss,” consisting of spans, varying from 30 to 260 
feet in clear, which in several instances have been (as will 
hereafter be shown) subjected to the most severe and extra¬ 
ordinary practical tests. It is not, therefore, certainly too 
much to expect, that the engineering profession will read¬ 
ily concede that this plan of structure is, at least, no longer 
a matter of experiment. And if the reader will carefully 
peruse the following pages, which contain evidence of an 
incontrovertible character in support of this position, he 
cannot fail to discover, that the claim for this particular 
form of truss is not made unadvisedly. 

Fig 1 is what is known as the “ Burr Bridge.” It is 
composed of lower and upper chords, and posts and braces. 
The posts are framed into the chords, and the braces are 
framed into the posts. Arches are placed on each side of 
the truss, securely fastened thereto, and extending below 
the lower chords, abut against the masonry. 

This form of truss was extensively used throughout the 
United States previous to the introduction of railroads. 
Many spans were of great length, and in cases where the 
arches were large, and the masonry sufficiently permanent, 
this bridge was comparatively successful. Much difficulty 
was, however, experienced, by reason of the absence of 
counter braces. A moving load produced a vibratory and 
undulating motion, tending to loosen the connection of the 
timbers, which generally resulted in failure. 

Many of the first railroad bridges in this country were 
built upon this plan, but much greater difficulty was found 
in adapting it to the use of railroads, than had been pre¬ 
viously experienced in its use upon common roads. This 















6 

difficulty arose from, 1st, the practical impossibility of per¬ 
fectly combining the action of the arch and the truss (each 
system, of itself, being insufficient to carry the whole 
load) ; and 2d, the absence of counter braces. These de¬ 
fects, clearly apparent in their use on common roads, were 
greatly aggravated under the increased and concentrated 
nature of the weight, and the rapid transit of trains on rail¬ 
roads. It is true, they were obviated in part by adding 
largely to the amount of material in the structures ; but, 
as the difficulty was inherent in the plan, violent contor¬ 
tions in shape could not be prevented, and these in time 
caused failure. 

These remarks are intended to apply to spans of consid¬ 
erable length, as experience has proved that plans of even 
an inferior grade may be measurably successful in spans of 
ordinary length ; whereas, nothing short of the most judi¬ 
cious distribution of material will insure permanency, in 
cases where long spans are indispensable, and any arrange¬ 
ment which can be made permanent in the latter case, 
must certainly prove so in the former. 

It is worthy of remark here, that this particular combi¬ 
nation of the arch with the truss, is even now with some, a 
favorite idea, but it is believed that its wannest advocates 
will be generally found among those whose opportunities 
for practical investigation have been limited, and that it is 
only necessary that the question be properly presented to 
them, to produce a change of views in respect to it. 

This partiality for the combination of the arch and the 
truss is attributable partly to the tact, that the simple truss 
has in many instances failed, arid as a last resort, the arch 
has been added, of such dimensions and strength, as to be 
competent to carry the truss and load, the truss serving 
only as a stiffener to the arch, while the latter, thrusting 














7 

upon the masonry, has sustained the whole weight. Be¬ 
sides, to the casual observer, who has never studied bridge 
construction, this combination presents at least an appear¬ 
ance of great strength and solidity, which do not in fact 
exist. 

That the simple truss without the arch has failed in some 
instances, is unquestionably true ; but while many of' these 
failures have been caused from inattention to, or ignorance 
of, the laws regulating the composition and resolution of 
forces, by far the greater number have arisen from the in¬ 
ferior quality, or lack of the requisite amount of material, 
or from inferior workmanship, or perhaps from all com¬ 
bined, either of which would produce the result, for it is 
the perfection of all these points which constitute a reli¬ 
able structure. 

The acknowledged failure of the “ Burr Truss,” as ap¬ 
plied to railroad purposes, led to the invention of several 
other plans, all of which were based upon the abandon¬ 
ment of the arch, and were aimed at perfecting a truss, 
which, of itself would be sufficient to meet the emergencies 
of the case. This was in pursuance of what was consid¬ 
ered a very reasonable hypothesis, viz.: that one sj^stem 
properly proportioned, must prove much superior to any 
method or arrangement in which the attempt was made to 
combine two distinct principles, in their nature heterogene¬ 
ous. 

Among the most prominent plans presented at this junc¬ 
ture of affairs, was one invented by Col. Stephen H. Long, 
of the United States army, a gentleman of great research 
and high scientific attainments. This plan of bridge was 
composed of lower and upper chords, posts and braces, 
similar in outline and general arrangement to the “ Burr 
Truss,” but differing from it in detail. An efficient system 










8 

i 

of counter braces was introduced ; these were made ad- 

i 

justable by wooden wedges, as were also the sustaining 
braces, by means of which any desirable elevation or deflec¬ 
tion might be given to the truss. This plan of truss was 
rigid to a degree not previously attained; and to such an 
extent was this true, that when properly adjusted, no per¬ 
ceptible deflection was produced by the passage of the 
load. 

It is believed that to Col. Long belongs the credit of 
having first discovered the proper office of a counter 
brace ; as about that time he made the statement, then 
considered paradoxical, and even at this day esteemed so 
by many, viz : “ That by means of the counter braces, 
a bridge could be so adjusted in itself, that the passage of 
the load would not produce any additional strain upon 
the sustaining braces, but would relieve the strain upon 
the counter braces equal to the weight passing over it.” 

Herman Haupt, Esq., civil engineer, in his admirable 
treatise on bridge construction, reiterates this in substance; 
and although the correctness of this theory has been fre¬ 
quently called in question, by those who have not given 
the subject much thought, yet its truth is now readily con¬ 
ceded by all who have had opportunities for investigation. 

This introduction of counter braces was an important 
step in the progress of bridge construction, which merits 
more than a passing notice, and will therefore be again re¬ 
ferred to. 

Col. Long’s bridge, taking into consideration its merits 
as a simple truss , is undoubtedly, in a theoretical point of' 
view, one of the most nearly perfect structures ever in¬ 
vented. It was, however, found difficult to keep it in 
adjustment, in consequence of the great shrinkage of the 
wedges and other timbers of the truss, and thus the par- 

















V 




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9 

ticular method adopted to produce such a desirable result, 
doubtless contributed to prevent the general introduction 
of the bridge. 

The invention of what is known as the “ Howe Bridge,” 
which has been extensively used throughout the United 
States, soon followed. In this, as in Col. Long’s bridge, 
the idea of combining the arch with the truss was originally 
abandoned, for reasons heretofore given, and it was believed 
that this simple form of truss would prove equal to any 
reasonable requirement. 

In the “ Howe Bridge,” the posts used in the Burr and 
Long bridges are dispensed with, and iron rods substituted 
(see Fig. 2), by means of which any desirable “ camber” 
may be given to the truss, thus overcoming the practical 
difficulty previously experienced in the adjustment of Col. 
Long’s bridge, by the use of wooden wedges, as above re¬ 
ferred to. 

This method of producing camber is certainly an im¬ 
provement upon the means adopted in the Long bridge, for 
that purpose, but is much inferior to the latter in its 
method of counter bracing, in that they are not adjustable, 
and perform a negative , rather than a positive duty, as there 
may be occasion to show hereafter. 

The “ Howe Bridge’’ is composed of lower and upper 
chords, braces and counter braces, vertical rods, and cast 
iron “ bearing blocks.” The braces abut upon the “ bear¬ 
ing blocks,” which pass through the chords in such a 
manner as to permit the rods to bear directly upon them. 

This form of bridge was first extensively used in the 
New England States, and was subsequently introduced 
generally ; and while it must injustice be admitted, that 
no plan of bridge, the same length of time before the pub¬ 
lic, has given so much general satisfaction, yet it cannot be 



10 


denied, that the energy, ability and influence of the parties 
controlling its interests, have in some degree at least, con¬ 
tributed to this result. 

The popular success of this bridge has induced parties 
from time to time to attempt improvements in its details, 
leaving the general principle the same; but the improve¬ 
ments claimed have either resulted in an infringement of 
the original patent, or have turned out comparatively 
worthless. 

Spans of considerable length were built upon this plan, 
but experience proved that even this truss—like all others—r 
had its limit, beyond which it could not be safely ex¬ 
tended. 

In the progress of Railroad enterprises, in order to save 
large expenditures of money for masonry, longer spans 
than had been previously used became desirable, and in 
certain locations absolutely indispensable; besides this, 
locomotives were largely increased in weight, to meet the 
demands of traffic, and furnish a more economical mode of 
working, and thus arose the necessity for the adoption of 
some other expedient to meet the increased requirements 
of bridges. As all had been done by way of improving this 
truss that mechanical skill could devise, and which an ex¬ 
tensive practice had amply afforded, it became evident, that 
some radical change must be made in its arrangement, to 
enable it to meet the exigencies of the case. 

In this emergency the arch , heretofore condemned in the 
“ Burr Truss,” was again resorted to, for it had been 
proved from the experience which its use in that truss 
had afforded, that an arch of sufficient size abutting against 

o O 

permanent masonry, would place the truss in a position of 
secondary importance. 

It may be considered excusable here, to venture the re- 


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11 

mark, that the adoption of such a radical change—although 

clearly a matter of necessity—was nevertheless a virtual 

/ . 

surrender of what had been previously claimed for this plan 
of truss, and had been in a great measure the means of giv¬ 
ing it an enviable reputation, and was undoubtedly a step 
taken in the wrong direction. It may also be remarked, 
without intending offense, that if the combination of the 
arch and truss was considered inconsistent, when previous¬ 
ly practised in the days of the Burr Bridge, it is difficult 
to perceive why a simple change of circumstances should 
have led to its approval now. 

It will be observed that the arch of the Burr Bridge, Fig. 
1, abuts upon the masonry in precisely the same manner as 
the arch of what is denominated the “ Improved Howe 
Truss,” Fig. 3, and the difference between the two consists 
simply in the mode of connection with the truss, and not 
in any change of principle, or method of action. 

It will be seen that the Burr arch is securely fastened to 
the posts and braces of the truss, forming a solid unadjust- 
able mass. In Fig. 3, the arches are not fastened to the 
braces or rods, but have an independent connection with 
the lower chord .of the truss, by means of rods radiating 
from the former to the latter. By this method it was sup¬ 
posed that any desirable adjustment could be effected, and 
that the strain could be put upon either system, or equally 
upon each. 

This new arrangement, although plausible in theory, is 
found impossible in practice, for the following reasons : 

1st. The rods from the arch to the lower chord are of 
various lengths, consequently their contractions and expan¬ 
sions must vary proportionately. 

2d. Not a single rod in the arch is of the same length as 
those in the truss, hence the expansion and contraction of 
















■ - -—. . ■ ■ « ' .. . . . ' ~ - --- — ■' - 

12 

the rods in the truss will vary from that in each and all the 
rods connecting the arch with the lower chord. 

3d. This combination is exceedingly liable to maltreat¬ 
ment, from the careless or ignorant. 

4th, And even if it were everything in practice that is 
claimed for it in theory (which is not the fact), it involves a 
constant expenditure for adjustment, which must continue 
during the existence of the bridge itself. 

The Burr Truss, Fig. 1, with all its defects, can be made 
superior by far to the “ Improved Howe Truss,” Fig. 3. 
For in the former, there may sometimes be a yielding and 
compression between the parts of the truss and those of the 
arch, producing a certain degree of united action ; while 
in the Howe Truss, everything depends upon the length of 
the rods, which must always change with the temperature, 
and thus render an approach even to perfect adjustment, a 
matter of extreme delicacy. 

But in either Fig. 1 or Fig. 3, it is clearly evident that, 
in order to have a structure absolutely safe, the arch and 
the truss—each of itself, independently of the other—should 
be of sufficient strength to sustain the whole load, that the 
strain may be borne alternately by each separate system. 

Herman Haupt, Esq., in his “ Treatise on Bridge Con¬ 
struction,” page 174, giving a calculation of the strength of 
the Howe Truss and Arch Bridge, built across the Susque¬ 
hanna River, on the Pennsylvania Railroad, in spans of 150 
feet clear, remarks as follows: 

“ Before we proceed to calculate upon the parts which 
compose this truss, it is necessary to state distinctly the 
principles upon which such calculation must be made. 

“ It is evident that where two svstems are connected in 
the same truss, each capable of opposing a certain resist¬ 
ance, it will be very difficult so to proportion the weight 























































































































I 


13 

upon each, that the load will be in proportion to the sev¬ 
eral portions. If, for example, a truss be constructed, and 
the falseworks removed before the introduction of the 
arches, if the latter be bolted to the posts, the weight of 
the whole structure is sustained by the truss itself, and the 
arches will not bear a single pound, unless they are called 
into action by an increased degree of settling in the truss ; 
but if the bottom chord of the truss is connected with the 
arches by means of suspension rods with adjusting screws, 
the whole truss may be raised upon the arches, and in this 
case the latter will bear the whole weight, and the former 
none. 

“ Again, if we suppose the arches to be connected with 
the truss before the removal of the falseworks, and the 
joints be equally perfect in both systems, there is a pros¬ 
pect of a more nearly uniform distribution of the load, but 
even in this case we cannot tell what portion is sustained 
by each system, because this will depend upon their rela¬ 
tive rigidity. If, for example, one of the systems should 
experience double the deflection of the other, with a given 
load, the less flexible would sustain twice as much as the 
other when combined, provided they are so nicely adjusted 
as to bear equally when unloaded, except with the weight 
of the structure. In practice, the most convenient way of 
securing an equal bearing, appears to be to remove the 
falseworks before the arches are introduced. After the 
arches are in place, examine the level of the roadway, and 
screw the nuts of the suspension arch rods, until the truss 
begins to rise very slightly. As there is necessarily a cer¬ 
tain degree of elasticity in the truss, it will then be certain 
that both systems are in action. With all these precau¬ 
tions, there are still difficulties in v estimating the exact 
strain upon the parts of a bridge which is sustained by two 











14 


V 


different systems, for there may be unequal settlement, and 
the adjustment, however accurately made in the first place, 
may not long continue. It can, it is true, be tested at any 
time by unscrewing the suspension bolts until the truss 
ceases to settle, and then screwing up again until the truss 
begins to rise, but it will generally happen that after a 
bridge has been a long time in operation, the two systems 
bear very unequal proportions, and when the truss itself is 
not so constructed as to be susceptible of adjustment, the 
arch almost always sustains the whole weight of the 
bridge and its load. 

“ These, and many other considerations, have led the 
writer to the conclusion, that the best method of construct¬ 
ing bridges, is to place entire dependence upon the arch, 
using the truss merely as a system of counter bracing, and 
a support for the roadway.” 

During the year 1847, the New York and Erie Railroad 
being then in progress of construction, and requiring a 
number of important bridges, a general competition was 
invited by its managers. Models and plans of the various 
bridges in use were placed before the engineer department; 
minute examinations were caused to be made of bridges 
built after the several plans presented. Ample opportuni¬ 
ties were afforded for explanation and discussion of the 
merits of each, and every means resorted to, to elicit useful 
information on the subject. 

Among the competitors were advocates for the simple 
truss, without the arch ; others claimed superiority by the 
peculiar method adopted in combining the two systems ; 
while others contended that the arch alone, if properly 
confined, was much superior to either plan. 

After a full discussion of the subject, occupying a con¬ 
siderable length of time, it was found that there had been 



15 


failures in all the plans of trusses presented, and that the 
arch, as a last resort, had been added. 

It was given as the opinion of the engineer department, 
that the arch in combination with the truss, should be 
adopted in some form,—that although this combination 
had not been successful in all cases, yet its failure resulted 
either from an imperfect method of adjusting the two sys¬ 
tems, or from an improper distribution of* the material. 

It was therefore determined, after mature deliberation, 
to adopt the plan shown at Fig. 4. 

During the year 1848, the writer was contractor for 
bridges upon the road referred to, and was subsequently 
appointed by the company to take charge of the bridges 
and other structures upon the line. This position was 
accepted the more readily, because it offered an excellent 
opportunity for theoretical and practical investigation in 
the art of bridge construction, for which lie felt himself 
somewhat prepared, by a close application to mechanical 
pursuits for a number of years previous. There was not 
at that time before the public, any work upon the theory 
and practice of bridge construction. Many theories were 
advocated, which a more thorough investigation has proved 
fallacious, and generally the self-styled “ practical man” 
adopted certain methods of construction simply because he 
had done so before, and was as dogmatical in expressing his 
opinion, as he was ignorant of many of the principles in¬ 
volved. 

Upon examining the plan, Fig. 4, it will be seen that the 
arch rests upon the lower chord of the truss. It is com¬ 
posed of six pieces, each 4X1:2 inches, making the sectional 
area L2X24 inches. The lower chord is composed of four 
pieces, each 6X12 inches; sectional area, 12x24 inches. 
The upper chord is made up of three pieces, each 6X12 










16 

inches; sectional area, 12X18 inches. The posts are 8x12 
inches, and the sustaining braces are six inches in thick¬ 
ness, and vary from eight to thirteen inches in width. The 
chord pieces are closely fitted together ; “ packing blocks” 
are inserted between, and the whole secured with vertical 
bolts. The posts are “ locked on” to the outside of the 
chords and arch, and are fastened together by means of 
bolts, which pass over and under the same : they are not 
framed into the chords or arch, as thev were intended to 
be moved horizontally in either direction, and are so framed 
as to permit a large wooden key to be inserted above and 
below the arch. Large wooden keys are also inserted back 
of each pair of posts, passing through the chords, and re¬ 
ceiving the entire thrust of the braces. The arch abuts 
upon large cast iron “ shoes,” which are laid upon the 
chord, and attached thereto by means of heavy wrought- 
iron links, which pass around projections on the “ shoes,” 
and around castings inserted in and passing through the 
chord, thus transferring the thrust to the latter. The 
counter braces are adjusted by means of wooden keys. By 
this method, it teas supposed that a perfect adjustment could 
be attained ; that, by loosening the keys under the arch, 
and driving the keys over the same, the truss could be 
raised, and the whole strain placed upon the arch ; that, 
by driving the keys back of the posts, the latter could be 
made to slide horizontally upon the chord and arch, and 
thus the whole, or any portion of the strain, be put upon 
the truss. 

Very soon after these bridges were brought into use, it 
became evident that however plausible this method of ad¬ 
justment might seem in theory, it was an entire failure in 
practice. The sudden shrinkage of the arch, posts, and 
keys, caused an almost immediate deflection of the whole 



17 


structure. An attempt was made to restore it to its orig¬ 
inal position, by driving the keys referred to ; but as many 
of the latter had yielded, and the fibres of the timber had 
become completely interlocked, this was found to be im¬ 
possible. 

The next step taken, was to raise the whole structure 
upon false works, when many of the keys were found to 
be so crushed, as to render new ones necessary. 

Very soon the lower chord gave evidence of failure, 
caused by the great thrust of the arch and braces ; this 
again produced deflection, and as many of the pieces com¬ 
posing the lower chord had pulled apart, the case became 
somewhat critical. The system of counter bracing being 
very imperfect, the deflection changed with the position of 
the load; many of the keys became loosened, and in some 
cases, the vertical vibration was such, as to shake them out 
altogether. 

These facts were duly reported to the management, ex¬ 
aminations were made, various means were resorted to, to 
strengthen the bridges; where practicable, spur braces 
were introduced, from the masonry to the lower chord ; 
castings were inserted vertically through the latter, to pre¬ 
vent the pieces of which it was composed, from sliding on 
each other, and other expedients were tried, which it is not 
necessary to state here. Suffice it to say, that after a large 
amount of money had been expended in endeavoring to 
make these bridges serviceable, it seemed to be clearly 
evident that nothing could be done to prevent their 
failure. 

These remarks apply more especially, to spans of con- 

3 










18 


siderable length, although all the bridges built upon this 
plan, were troublesome and expensive. 

Thus, this method of combining the arch and truss, like 
all previous attempts, proved a failure, and even if a per¬ 
fect adjustment of the two systems were possible, in this 
particular case at least, the structure must have failed, in 
consequence of the great disproportion existing between the 
amount of material subjected to a crushing strain, and the 
imperfect application of the material in the lower chord, 
rendering it incapable of resisting the tensile strain thrown 
upon it by the united action of the arch, upper chord, and 
braces. 

This question of combining the arch and truss, has been 
thus treated minutely and particularly, for reasons that will 
be apparent as we proceed in the discussion of this subject. 
In doing so , facts rather than theory, have been presented, 
and these facts can be substantiated, by those who will 
take the trouble to examine the class of structures referred 
to. 

The failure of these bridges caused great alarm to the 
management, direction was immediately given to sup¬ 
port by means of “ trestle work,” such as exhibited evi¬ 
dence of failure, until some means could be devised to meet 
the exigencies of the case. As the locomotives upon this 
road were generally much heavier than those in use upon 
roads of a narrower gauge, it seemed doubtful whether the 
most approved bridge structures upon the latter, would be 
successful upon this. 

At the suggestion of the writer, permission was given 
him to institute a series of experiments with models, in 










19 

order if possible, to perfect a plan of bridge which could 
be relied upon, and which might be substituted for those 
then upon the road. 

Under this authority, experiments were commenced with 
models of the various bridges in use, which models were in 
all cases loaded to the breaking weight, thus any inherent 
defect, was clearly, and unmistakably made manifest. 

These experiments were, at the outset, based upon the 
abandonment of the arch, for reasons herein stated, and it 
was determined to direct the investigation to the attain¬ 
ment of some simple form of truss, which would be suffi¬ 
cient. 

As a basis of operations, a model of the plan of bridge 
last described was selected; this model represented a span 
of 150 feet in clear, and was built upon a scale of J inch 
per foot, the details being in every respect the same as in 
the full sized bridge. 

The apparatus for loading, may be described as follows : 

Floor beams were placed one on each side of all the 
posts in the trusses, and as nearly thereto as possible, ex¬ 
tending outside of the latter, to permit a platform to be 
suspended therefrom, by means of iron rods, two to each 
pair of floor beams—upon this platform was placed the 
load. By this arrangement, the load was suspended 
from each pair of posts in both trusses. 26,000 pounds of 
castings were prepared in pieces weighing 100 pounds each, 
these pieces were all of the same dimensions, and their 
lengths were equally divided by a mark through their cen¬ 
tre. In order that the load might be applied equally upon 
both trusses, a line was drawn longitudinally in the centre 



20 


of the platform, to which the centre marks in the weight 
were made to conform, the platform was then loaded with 
one tier of weights, from end to end, thus distributing the 
load equally between the supports, the deflections caused 
by the load were accurately noted, and much care was 
taken to ascertain the effect upon the various portions of 
the trusses; the weight was then increased until the model 
was broken, when it was found that the arch, upper chord, 
and braces, contained sufficient material when fully 
brought into action, to separate by tensile strain, lower 
chords of at least double the sectional area, of those used, 
and constructed in the same manner. 

This result furnished a striking illustration of an im¬ 
proper distribution of material in the bridge, and afforded a 
hint of what might be reached by a series of judicious and 
well conducted experiments. 

Experiments with models have been objected to, on the 
ground that they afforded no data, from which the strength 
of a full sized structure, can be calculated. 

It is true, that to ascertain the number of 'pounds a full 
sized bridge would sustain, from any experiment made 
with a model of the same, would at least be difficult, for 
the reason that a model built upon a scale of one inch 
per foot, while it measures one twelfth the length and height, 
and each timber measures on its surface one twelfth the 
dimensions of that in the full sized bridge, the sectional 
area is 144th, and the cubic contents only one J,728th 
part, to which may be added the fact, that models are 
usually built with more care than full sized structures. 

It is nevertheless evident that models of different struc- 










tures—of the same length of span, built upon the same 
scale—of like kind, and amount of material, and of equal 
perfection in workmanship, when submitted to the same 
test, will not only demonstrate the relative value of particu¬ 
lar combinations, but will as clearly exhibit the nature and 
intensity of the destroying forces in each . 

It was therefore determined to try the various combina¬ 
tions of bridge trusses in use. That these experiments 
might be fully relied upon, to show the relative merits of 
each, it was decided to use in all future models the same 
amount of material in value , as was contained in that 
already tested; for this purpose, it was necessary to make 
a correct estimate of quantities, and to place a value upon 
timber and iron, by which the whole amount of material 
might be regulated. 

As these trials were strictly private, there could be no 
inducement to favor one plan of truss, to the injury of any 
other. 

Where great dependence was placed upon iron rods, the 
wire representing the same in the models, was of the best 
charcoal iron, manufactured especially for these experiments, 
and all other precautions were taken to ensure a fair and 
impartial trial, of all the various forms of trusses before the 

Persons interested in several of the plans were kind 
enough to furnish bills of material distributed according to 
their best judgment, the whole value not exceeding the 
prescribed limit. 

A close inspection of the cause of failure in any particu¬ 
lar form of truss may be obviated in a repetition of the ex- 















22 

periment, by an improved distribution of the same amount 
of material. 

In order to illustrate this, it may be stated that the first 
model of a well-known structure, was broken with a pres¬ 
sure of about 11,900 pounds ; in this trial, failure was 
caused by a separation of the lower chords, while the 
other portions of the trusses remained comparatively per¬ 
fect. Another model was built upon the same plan, and 
at the suggestion of parties directly interested, the trusses 
were reduced in height, the span remaining the same, the 
sectional area of the chords was increased, as were also the 
sustaining braces and their connections; this model also 
failed from a separation of the lower chords, and was 
broken with a pressure of about 11,000 pounds, establish¬ 
ing the fact, that an increase in the dimensions of the parts 
failed to compensate for the reduction in height of 
trusses. 

Although a third experiment was not made, to test the 
full capacity of a given amount of material, in this particu¬ 
lar form of truss, enough was accomplished to show that in 
general practice, the parts had not been so proportioned, as 
to ensure the greatest attainable strength; this was made 
positively certain by subsequent experiments upon sub¬ 
stantially the same combination. 

These experiments, it is believed, were prosecuted to an 
extent unprecedented in the history of this branch of the 
engineering profession, and at a larger expenditure both in 
time and money, than could reasonably be expected 
through unaided individual enterprise. 

Models of every conceivable combination of wood and 



23 

iron were made and broken, the material was in some in¬ 
stances increased to a degree that might be deemed im¬ 
practicable, and while a minute description of each particu¬ 
lar experiment might serve to amuse the reader, little use¬ 
ful information would be elicited in doing so. Suffice it to 
say, that many of these models were as fallacious in con¬ 
ception, as they were expensive in execution. 

Nevertheless, however absurd and unnecessary many of 
these experiments now seem to he , they served at the time 
to undeceive the writer, and had the effect of correcting 
many preconceived opinions ; but while acknowledging 
this, he must be permitted to remark, that he is occasion¬ 
ally amused, even now, in witnessing combinations, sub¬ 
stantially the same, executed on a larger scale, and in 
actual practice , and may be allowed to offer by way of apol¬ 
ogy, that the process by which he arrived at the result, 
although somewhat costly, was by far the least expensive 
of the two. 

Much labor has been expended in perfecting what was 
believed to be vast improvements in bridge construction, 
and to which great importance has been frequently at¬ 
tached, which although sometimes creditable to their orig¬ 
inators, really amounted to nothing more than simple 
alterations in detail, the general action of the truss being 
the same. 

In order to simplify and make clear the real points of 
difference existing in the combinations of the various plans 
of trusses, of the same general outline, it may be stated 
that the material composing any bridge truss , whether of 
wood or iron, or of both, is subjected either to tension or 



24 


thrust , and it is upon the proper application of these ele¬ 
ments, together with a judicious distribution of the mate¬ 
rial, rather than upon any difference in detail, that the per¬ 
fection of any bridge structure depends ; this may be illus¬ 
trated by reference to Figures *5, 6, 7. 

j Fig. 5. 



Figure 5 is the truss of the Burr Bridge; in this the up¬ 
per chord and braces are acted upon by thrust, and the 
lower chord and posts by tension. 



Fig. 6 . 


Figure 6 is the Howe Truss, without the counter braces ; 
in this also, the upper chord and braces are subjected to 
thrust, and the lower chord and vertical rods are acted up¬ 
on by tension. 



















































25 



Figure 7 is a, plan of truss sometimes used, the counter 
rods being omitted; in this the upper chord and vertical 
struts are subjected to thrust, and the lower chords and 
diagonal rods are acted upon by tension. 

Upon a comparison of these plans it will be discovered 
that the variations between the Burr Truss, Fig. 5, and the 
Howe Truss, Fig. 6, consists in the use of vertical rods and 
bearing blocks in the latter, instead of vertical posts in the 
former, both having precisely the same duty to perform. 

It will also be seen that Fig. 7 varies from Fig. 6, in that 
the rods are placed diagonally instead of vertically, chang¬ 
ing the element of thrust from the diagonal braces in the 
latter, to the vertical struts in the former, and transferring 
the element of tension from the vertical to the diagonal 

line. ** 

As has already been stated, much importance is some¬ 
times attached to just such modifications in detail as e . st fn* 
Figures 5, 6, 7, while the nature and intensity’of the destroying 


forces are the same and equal in each . 

This has been proved by actual experiment, as follows : 
Models were built, one on each plan, of equal length and 
height of trusses, containing the same sectional area and 
kind of material in chords and braces, and of equal perfec¬ 
tion in details and workmanship, when it was found that 

4 





















26 


the real difference in strength was unappreciable, and it 
may be well to add, that any given amount placed upon 
each, in progress of the experiments, presented precisely the 
same characteristics and contortions in shape, until final 
failure took place. 

All bridges having their chords parallel, irrespective of 
the particular method adopted in combining them, and re¬ 
gardless of the amount of material used in their construc¬ 
tion, when loaded to nearly the point of fracture, present 
somewhat the appearance of Figure 8, the greatest deflection 

j Fig. 8. 



being invariably at points near the abutments. This will 
be understood by the statement, that the vertical strain is 
increased, as the distance from the centre, to the ends of 
the truss ; at the centre the vertical strain is nothing, and at 
each end of the truss, it is equal to one half the weight of 
the structure and its load ; this may be illustrated by refer¬ 
ence to Fig. 8. Let it be assumed that weights A, B, C, are 
each equal to four tons, one being suspended to each verti¬ 
cal rod in the truss, it will be seen that brace No. 1 will 
sustain weight A, four tons; brace No. 2 will sustain weight 
A, four tons, and weight B, four tons, which is eight tons; 
brace No. 3 will sustain weights A, and B, each four tons, 
and also weight C, four tons, in all twelve tons, or one half 
the whole load applied. From this it follows that brace 






























27 


No. 3 should be twice the strength of brace No. 2, and 
three times the capacity of brace No. 1, and the same rule 
is equally applicable to the rods connecting the chords. 
This disparity of pressures will be greater as the spans are 
increased in length. 

To remedy this defect various expedients have been re¬ 
sorted to; heavy bolsters have been used, extending some 
distance from the points of support, spur braces have been 
applied from the masonry to the lower chords, and in some 
cases, they have been extended to the upper chords ; arch 
braces have also been introduced, from the lower to the 
upper chords, and iron rods have been added, which were 
placed diagonally from the ends and top of the truss, to a 
point upon the lower chord at some distance from the ma¬ 
sonry. 

It is worthy of remark here, that although this peculiar 
mode of action has been long known, as is evident from the 
various means adopted to overcome the same, it is never¬ 
theless remarkable, that it is only of late years, that the 
real cause has been discovered. 

Upon examining many structures now in use, one of two 
things will generally be found, viz., that the sustaining- 
braces and their connections are larger toward the centre 
of the bridge than required, or that they are smaller towards 
the ends than they should be, and it is only necessary to 
keep in view this simple principle of the increased vertical 
strain, in order to arrive at correct conclusions in regard 
to the merits of any bridge truss, so far as this particular 
point is concerned. 

A point has now been reached in the discussion of this 













28 


question, making it necessary to state, that after having 
spent much time in efforts to perfect some truss of this 
particular outline, and much had been accomplished in 
ascertaining the nature and intensity of the various forces 
involved, which led to a more judicious disposition of the 
parts, and by which a large increase of strength had been 
effected with the original quantity of material, it was 
nevertheless evident that there existed defects, which were 
inherent, and which no amount of material, however large, 
the most perfect distribution of the same, nor the highest 
degree of workmanship, could possibly obviate. 

Prominent among these defects, was the absence of any 
mode of action, by which the truss itself could be made to 
counteract the increased vertical strain toward its ends. 

The use of spur braces, arches, tension rods, and other 
extraneous expedients, has already been referred to, all of 
which having failed to meet the requirements of the case, 
as indeed, every appliance must , which does not form an 
integral portion of the truss itself. 

All bridges having their chords parallel, as has already 
been stated, exhibit the same uniformity of' action, which 
may be illustrated by reference to Fig. 9, in which A, A, is 


Fig. 9. 

O 




























29 

upper chord ; B, B, lower chord ; C, C, tension rods; D, D, 
braces; E, E, E, struts ; W, weight. 

The strain produced by the weight W, is transferred by 
means of the strut E, to the tension rods C, C, and from 
thence to the braces D, D, and through the latter to the 
lower chord, A, A, D, D, and E, E, E, being subjected to 
compression, and B, B, and C, C, to extension. When a suf¬ 
ficient weight is applied to any truss of this outline, to 
cause deflection below a straight line, the upper ends of the 
braces D, D, are made to approach each other, and the dis¬ 
tance between the ends of the upper chord is diminished, 
and when great deflection is produced, the upper ends of 
the braces D, D, will have described arcs of a circle down¬ 
wards , as represented by the dotted lines F, F, the radius 
of which being the length of the braces D, D. 

To remedy this and other defects, the “Inflexible Arched 
Truss” was invented. As an illustration of its action, see 


Fig. 10. 



Figure 10, in which A, A, is lower chord; B, B, upper chord; 
C, C, tension rods; D, D, braces; E, E, struts; and W, weight. 

Upon an inspection of this figure, it will be seen that 
any deflection produced upon the centre of the arch, by 
means of the weight W, will cause the points B, B, to sep- 
































































30 

arate, by thrusting outward , and in the direction of the ends 
of the truss, producing an upward movement of the upper 
chord, at the ends of the braces D D, the latter describing 
arcs of a circle upward, in the direction of the dotted lines, 
c, c, and from thence will be communicated by means of 
the tension rods C, C, to the centre of the lower chord, 
raising the latter at the point where the rods C, C, meet. 

By removing the weight W, and inserting a vertical 
strut, as represented by the dotted lines F, the upward 
movement of the chords will be arrested by the weight W. 
This peculiar action may be described as follows. 

Any deflection produced in the centre of the arch will 
cause an outward , and consequently, an upward force, at the 
upper ends of the braces, which, by means of the tension 
rods and strut, is transferred directly back to the under side 
of the arch, producing an upward force at the latter point, 
equal to the original downward force applied on top of the 
same. 

This combination of forces is in agreement with a well- 
known law in philosophy, viz., when two forces of equal 
powers of resistance are opposed to each other, a state of 
rest is produced. 

For a further illustration of the action of this truss, see 


Ffo-. ii. 





























31 


i 


Fig. 11, in which A, A, are pieces of the lower chord, the 
centre being removed, B, B, upper chord, deflected to nearly 
a straight line, by the weight W. C, C, are braces which 
pass through the lower chord, and rest upon the masonry. 
D, D, are tension rods. It will be seen that the ends of the 
pieces of lower chord at E, E, are raised considerabty above 
a horizontal line. This upward tendency will continue 
until the upper chord is deflected below a straight line, 
when the action will be reversed, as has been shown at 
Fiu. 9. 

Fig. 12. 



Figure 12 exhibits the forces at a state of rest, in which 
A, A, are portions of the lower chord ; B, B, upper chord ; 
C, C, arch braces, which pass through the lower chord, and 
rest in the masonry. D, D, tension rods; E, E, braces; W, 
weight. 

It will be seen that the strain produced by the weight 
W, is transferred to the lower chord by means of thrust 
upon the braces E, E, to the points F, F, and by means of 
tension on the rods D, D, to the points B, B, and from 
thence it is brought upon the arch braces C, C, which rest 
upon the masonry. 

In this manner, a perfect equilibrium of forces is effected, 

























32 


as it is evident that the point G, cannot change position, 
unless the points B, B, are thrust outward towards the ends 
of the truss, which must raise these points, this being pre¬ 
vented by the strain upon the points F, F, communicated 
by the weight W, through the braces E, E. 

Trusses built upon this principle, when loaded nearly to 
the breaking point, assume something near the shape 
shown at Fig. 13. By again referring to Figure 8, it will be 

Fig. 13. 



seen that the effect produced by the load upon Figure 13, is 
the reverse of that produced upon Figure 8, the latter being 
greatly depressed toward the ends ; the centre remaining 
comparatively stationary, while the former is raised toward 
the ends, and is deflected in the centre. 

It may be remarked here, that since the success of the 
“ Inflexible Arched Truss” has been generally conceded, 
parties in other bridge interests having witnessed its great 
strength and rigidity , in actual practice, but who, neverthe¬ 
less, seem to have been lamentably deficient in a proper 
understanding of the desideratum sought to be attained by 
its combination and arrangement, have in some cases 
adopted the arched upper chord, supposing , that in this 
alone , consisted its acknowledged superiority. That those 









































































































J 








! 

.* 

♦ 





i 




















































I 











































33 


parties may be undeceived who have been thus misled, it is 
only necessary that attention should again be given to 
Figures 8 and 13, where it has been seen that the contor¬ 
tions, under a great strain, are reversed. Still, it will not 
for a moment be contended, that the tensile force upon the 
lower chord is less in one case , than the other , and it must be 
evident upon the slightest reflection, that all possible 
advantage gained by simply adopting the arched upper 
chord, is nothing more than what may arise from an in - 
I creased height of truss, and not from any fancied, benefit accru¬ 
ing from a, mere change of outline, as precisely the same 
strength will be attained, in any truss having its chords par- 
| allel, provided its height is equal to that of the arched 
”pper c 1 3rd in the centre. 

The strength of either Figures 8 or 13 must be measured 
by the capacity of the lower chords to resist the tensile strain ; 
and assuming them to be of equal sectional area and tenac¬ 
ity, the one will not sustain a, pound more than the other. 

The following experiment made several years since, and 
the succeeding letter, will serve to show the original pur¬ 
pose in view, in adopting the arched upper chord, and in 
what manner great advantages may be derived from its use. 

In order to test the value of the combination of forces 
involved in the construction of the “ Inflexible Arched 
Truss,” the writer determined to institute an experiment 
upon such a scale, as would place its merits beyond contro¬ 
versy. 

A bridge of 140 feet clear span was being built, which 
was selected and prepared for that purpose (see Fig. 14), in 
which A, A, is lower chord; B, B, upper chord ; C, C, C, &c., 

5 




















34 

posts; D, D, D, D, sustaining braces; E, E, arch braces, which 
extend through the lower chord and abut against the ma¬ 
sonry ; F, F, tension rods. It will be observed that all the 
sustaining braces are left out in the centre of the truss, the 
lower chord being entirely dependent upon, and being sus¬ 
pended from the arched upper chord, between the points 
Gr, Gr. Both trusses were thus prepared. The floor tim¬ 
bers, lateral bracing, and track were completed. The 
bridge was then cleared from the false work, when it was 
found that the lower chord had assumed a straight line, 
very nearly. 

When the bridge was in this condition, a locomotive 
engine and tender was placed upon the centre, as repre¬ 
sented on the figure, which produced a deflection of about 
5^ inches at that point, and a corresponding upward move¬ 
ment toward the ends of the trusses. A locomotive engine 
was then placed at each end, as shown upon the figure, 
causing deflection thereat, which in turn produced an up¬ 
ward movement at the centre, and raising the locomotive 
placed there about 2| inches. 

The whole structure was carefully examined previous to 
removing the load, when it was found that the braces at 
the ends had bent laterally very slightly, the other portions 
of the trusses remaining perfect. 

From what has been stated explanatory of Figures 10, 11, 
12 and 13, it will not be necessary to give further reasons 
for the success of this experiment, as it must be evident 
that, so long as the arch braces were permanent, the 
deflection in the centre was caused principally by the up¬ 
ward movement at the points B B, requiring only addi¬ 
tional weight to be placed at the latter, to restore the 
structure to nearly the same shape it assumed previous to 
applying the load. 



35 

As the following letter, from a bridge builder of large 
experience, bears directly upon the point under discussion, 
it is here inserted. 

Ohio Sf Mississippi Railroad , > 

Lawrenceburg , Feb. 9 th, 1859. ) 

D. C. McCallum, Esq. 

Dear Sir : 

Your favor of the 2d inst., requesting my opinion of the 
“ McCallum Inflexible Arched Truss,” is received. 

In answer, I would say that I have been in charge of the 
bridges upon the eastern division of the Ohio and Mississippi 
Railroad for the past five years, upon which are 104 spans 
of your plan of bridge, varying in length from 35 to 210 
feet in clear, during which time I have had ample oppor¬ 
tunity to judge of the merits of the plan, as compared with 
other structures in use upon railroads, and I hazard nothing 
in saying, that after twelve years’ experience in bridge con¬ 
struction, I am fully satisfied that bridges built upon your 
plan will sustain a much greater load, are more rigid and 
durable, and require less care and adjustment, than any 
other form of bridge with which I am acquainted. 

In proof of the great strength of your plan of bridge, 
permit me to mention two instances, in which they were 
subjected to the most unprecedented tests : 

On the night of March 12th, 1858, an engine attached to 
a passenger train was thrown from the track about 300 feet 
distant from bridge No. 15. The train was running at high 
speed, and before it could be brought to a stand, the en¬ 
gine had reached the centre of the bridge, with all of its 
wheels off the rails, and upon it was piled one baggage car 
and two passenger cars. The concussion broke the lower 
chords of both trusses of the bridge, together with six posts, 



36 


twelve braces, and thirty floor beams. The engine, after 
having passed through the floor timbers, was arrested in 
its course by coming in contact with the track hungers 
and lateral rods. The lower chords being broken, the 
whole train was prevented from plunging into the stream 
by the sustaining power of the arch and arch braces alone. 
This proved the truth of what I have frequently heard you 
assert, that the arch and arch bracq^ alone were of suffi¬ 
cient, strength to sustain the whole structure, independent 
of any aid which might be derived from the tensile strain 
of the lower chord. I may further say, that when the 
bridge was in the condition as above stated, in order to 
raise the engine from its position, a heavy stick of timber 
was laid upon the arch chords across the bridge, to which 
blocks and fall and raising apparatus was attached, and 
although the latter was strained until it gave way, there 
was not the slightest evidence of failure in arches or arch 
braces, there being no supports from the bed of the stream 
whatever. This test surprised all who witnessed it, and 
who were not familiar with the principle of the bridge. 
Subsequent to this, two trains came in collision on bridge 
No. 54, smashing the engines together, and upon which 
was piled six cars, so high as to project above the upper 
chords. No timbers were broken, nor was any injury done 
to the bridge. 

In several instances, trains have been run over these 
bridges with portions of the engine or cars off the track, 
and in no case has any accident been caused by their fail¬ 
ure. From the above, and other points I might mention, 
I unhesitatingly recommend your bridge as the safest and 
most economical structure in use. 

Respectfully yours, N. S. Gardner, 

Supt. of Bridges Ohio § Miss. R. R 














37 


The above sufficiently explains itself, and the writer will 
add, that it is not only a complete vindication of the the¬ 
ory heretofore advanced in favor of the “ Inflexible Arched 
Truss,” but is an unanswerable refutation of the objection 
somewhat busily circulated by 'parties whose interests are not 
with this bridge, viz., that great danger is to be appre¬ 
hended from what they are pleased to term “ the immense 
thrust upon the masonry while the fact is known, that 
these bridges have been built upon wooden trestle piers 
and abutments with perfect success. 

For a full plan of this truss, the reader is referred to Fig. 
15, and also to the large engraving attached, where the de¬ 
tails are shown. Upon inspection, it will be observed that 
the sustaining principle is very much increased toward the 
ends of the truss, not only by a large addition to the amount 
of material at these points, but it will be seen also that 
the pannels become shorter as the vertical strain increases. 
The posts are placed upon lines radiating with the arch ; 
the braces form equal angles with the posts ; and in this 
way the latter are made to approach more nearly together 
toward the ends of the truss. 

The reader has already had sufficient evidence of the 
great strength of this form of truss, and it has also been 
shown, that the tensile strain upon the lower chord is much 
less than in any other known plan. In fact, the latter may 
be entirely severed, and the structure will still be competent 
to sustain a heavy load. In this, it differs from all other 
combinations. 

Upon referring to Figure 15, which represents a clear 
span of ISO feet, it will be seen that the arch braces which 
rest upon the abutments are extended to points on the arch 
about forty-seven feet from the abutments. From the top 
of each set of arch braces, running diagonally on each side 















38 


of the truss, are placed heavy suspension rods, which are 
connected with the lower chords 12 feet further from the 
masonry. Thus the bridge seat is substantially transferred 
to a point 47 feet towards the centre of the bridge, reduc¬ 
ing a span of 180 to 86 feet, so far as the tensile strain upon 
the lower chord is concerned. 

For this intermediate space of 86 feet, the arch beam is 
of sufficient strength to sustain the whole load, if required. 

Strength , however is not all that is required, for a Rail¬ 
road bridge especially, subject as it is to a moving load, 
there must also be rigidity , stiffness, freedom from vibration. 

A bridge may be strong yet flexible, rigid yet weak ; in fact, 
flexibility is incompatable with durability, the structure 
should be prepared at all times to receive its load, and should j 
not be permitted to change shape in the slightest degree, by 
its passage over it. 

To produce this result, an effective system of counter 
braces, is indispensable. 

The proper office of counter braces is frequently misun¬ 
derstood, as is evident from the manner of their application 
in many cases in which they are used as check braces only , 


Fig. 16. 





















39 

having a negative rather than a positive action ; this may be 
illustrated by Figure 16, which represents a short truss, in 
which A, A, is lower chord ; B, B, is upper chord; C, C, sus¬ 
taining braces; D, D, counter braces; E, E, struts; F, suspen¬ 
sion rods; W, weight. Assuming the length of the sus¬ 
taining and counter braces to be originally equal, when 
the load is applied as at W, the truss is deflected in con¬ 
sequence of the yielding of the braces C, C ; this has the 
effect of shortening the diagonals in the direction of their 
length, while the diagonals in the direction of the counter¬ 
braces are correspondingly lengthened; this, as will be seen, 
will leave a space between the ends of the latter, and the 
“ bearing block ” in the centre of the lower chord. 

When the truss is in this condition, if wedges are insert¬ 
ed between the ends of the counter braces and the lower 
chord, in such a manner as to fill up the whole space, it is 
evident that the weight W, may be removed without at all 
affecting the shape of the truss, the deflection originally 
produced by the weight W, being maintained by the coun¬ 
ter braces, the strain upon the sustaining braces and other 
portions of the truss remaining precisely the same as when 
the weight was suspended. 

Now suppose the original weight W, to have been ten 
tons, it is evident that as soon as it is removed, each coun¬ 
ter brace will be subjected to an upward thrust, equal to 
five tons, making ten tons, the weight of W. 

Now let there be a smaller weight suspended from the 
same point, say five tons , this weight will not produce any 
additional strain upon any portion of the truss, nor will the 
deflection be increased in the slightest degree; the only ef¬ 
fect produced by suspending the latter weight, will be the 
relief of the counter braces, equal to the difference between 
the first and second weights, viz.: five tons. The writer 



40 


has found it very difficult to explain this clearly in the 
course of conversation with some individuals, from the fact 
that weight and strain were confounded. Now it is true, 
when the original w eight was applied of ten tons, the abut¬ 
ments were loaded with just ten tons more than previously, 
and the truss was also loaded with ten tons more ; but when 
the w r edges were driven, and the weight removed, while 
the abutments were relieved of ten tons pressure, the truss 
still retained the original strain produced, the weight being 
required to produce the strain , the latter remaining after the 
former has been removed. 

In order to make a practical application of 1 e above, the 
following method of adjusting the “Inflexible Arched 
Truss,” is submitted. When these bridges are raised, it is 
usual to load them with a train of locomotive engines, at¬ 
tached closely to each other, and that greater weight may 

be obtained, the tenders are sometimes detached, and the 
bridge covered with engines only; with this load, the lat¬ 
ter is strained down to a perfect bearing in all its parts; by 
this means the whole structure is more or less deflected, 
while the counter braces are hanging loosely in their places ; 
if, therefore, when the bridge is in this condition with its 
load, the counter braces could be lengthened with consid¬ 
erable force, it would not recover its original shape upon 
removal of the load, but would be held down by the action 
of the counter braces to very nearly the same position as 
when loaded. In this plan of bridge, the lower ends of the 
counter braces rest in “ iron stirrups,” which are attached 
to the vertical ties or posts at a point near the lower 
chord by means of castings and nuts, by which they may 












41 


be lengthened several inches, in this manner they are made 
to perform a positive duty. When the bridge is adjusted as 
above, it is clear that a less load than that originally ap¬ 
plied cannot produce any deflection whatever, the only ef¬ 
fect of the passage of a train over it will be to relieve the 
counter braces, and will not add a pound pressure upon 
any timber of the trusses. 

In the arrangement of any bridge truss, the attainment 
v of the ludowing requisites is desirable : 

First , Such equilibrium of forces as will produce uni¬ 
formity of action. 

Second , S quantity and distribution of material, as will 

insure a large surplus of' sustaining principle, thereby guard¬ 
ing the structure against accident . 

Third, Perfect rigidity, that the combination in all its 
parts may have permanency equal to the durability of the 
material composing the same. 

Fourth, The arrangement of the parts should be such, as 
to be free if possible, from the necessity of adjustment. 

IIoW far the “ Inflexible Arched Truss ” meets these 
requirements may be learned by the following 

Extract from Appleton’s Mechanics’ Magazine and En¬ 
gineers’ Journal, April 1st, 1852, edited at that time by 
Julius W. Adams, Esq., a civil engineer of marked ability. 

McCallum’s Patent Timber Bridge. 

. 

“We have no hesitation in affirming, that of all the tim- 
her bridges patented in this country, there are none in 
poi^t of strength, economy (in which we include ultimate 

6 














42 

durability), facility of repair, and uniformity of action, to 
be preferred before this plan of bridge, lately patented by 
Mr. D. C. McCallum, of Owego, assistant engineer of way 
and structures on the New York and Erie Railroad. The 
bridge shown in elevation on plate No. 4 has been built 
lately for the New York and Erie Railroad at Lanesboro’, 
over the Susquehanna river, in the place of one built by 
ourselves, several years since, under the orders, and accord- 
ing to the plan, of the chief engineer of that road, but 
proved unequal to the duty imposed upon it, and its re¬ 
moval became a matter of necessity. We objected to the 
plan of the original bridge built in that locality, as we have 
ever done to any plan of bridge in which the attempt was 
made to unite the independent systems of arch and truss, 
and make the stability of the bridge dependent upon their 
uniformity of action. In this plan of Mr. McCallum, they 
are not independent as heretofore, but the action of the 
arch in the upper chord is made an integral part of the 
truss itself. And, instead of two systems acting unequally, 
and to the ultimate injury of the structure, we have the 
best features of both united in a manner which admits of 
entire uniformity of action. 

* 

“ On the 29th of January, 1852, a number of civil engi¬ 
neers, bridge builders, and mechanics of experience, assem¬ 
bled to make and witness such experiments as they might 
deem proper and satisfactory, for the purpose of testing the 
stiffness of the bridge across the Susquehanna river, near 
LanesboroY’ A committee was appointed to conduct the 
experiments, and report the facts, and from their report we 
quote the following : 



43 


“ This Bridge is known as McCallum’s ‘ Inflexible 
Arched Truss Railroad Bridge.’ It is constructed princi¬ 
pally of pine timber, with less than the ordinary proportion 
of iron rods, bolts and castings. The whole length of each 
truss is 200 feet, and has a clear span of 190 feet. ' 

“ The experiments consisted mainly in running from one 
to four locomotives across at different rates of speed, noting 
the effect, and observing particularly the deflection or set¬ 
tling of the trusses under various weights. 

“ The following are the weights of the engines and ten¬ 
ders used in the experiments. These weights include the 
usual complements of wood and water. 


Lbs. Lbs. Lbs. 

Engine No. 92. 49,510. Tender, 33,290. Total, 82,800. 


58. 

72,900. 

“ 35,000. 

“ 107,900. 

127. 

54,400. 

“ 36,300. 

“ 90,700. 

27. 

56,380. 

“ 36,630. 

“ 93,010. 


dross weight of four engines, 374,410. 


“ In running one of these machines back and forth at 
different speeds, no deflection could be detected by the 
naked eye, and a levelling instrument was used, to deter¬ 
mine the existence and extent of deflection. 

“ Upon placing first one engine and tender upon the cen¬ 
tre of the bridge, and noting the deflection ; then bringing- 
on successively the second, third and fourth, and arranging 
them as near the centre of the bridge as possible, the 
deflections were found as follows : 






Lbs. 

Feet. 

No. 



92. 

Gross weight, 82,800; or, 41 do tons, deflected. 

0,013. 



92 

, 58. 

« “ 190,700; “ 95 1 Vo “ 

0,038* 


92, 

58, 

127. 

“ “ 281,400; “ 140 do “ 

0,061. 

92, 

58, 

127, 

27. 

“ “ 374,410; “ 187 ro “ 

0,061. 













44 


“ On removing these engines from the bridge, it was 
found that the truss had entirely resumed its original level, 
showing that the elasticity of the timbers was unimpaired. 
The greatest weight put upon the bridge was a fraction 
less than one ton per foot of span,—was one and 1-10 tons 
per foot occupied by engines and tenders,—and was 2 1-10 
tons per foot occupied by engine No. 58. 

“ The plan of this bridge is the result of a series of ex¬ 
periments made by Mr. McCallum, who has, for several 
years, had charge of the bridges upon the New York and 
Erie Railroad. The experiments were highly satisfactory 
to all present, and we have no hesitation in expressing 
our conviction, that this structure is better adapted for 
railroad purposes, and particularly for bridges of great 
span, than any other in use. 

“ Mr. Hall, chief engineer of the Chemung Railroad, and 
well known as an engineer of experience in the construc¬ 
tion of bridges, addressed the following note to Mr. McCal- 
lum, on the subject of the strength of his bridge. 

“ ‘ Dear Sir : 

“ ‘ I regret that 1 was not able to meet with 

O 

the other members of the committee to report upon the 
experiments which we witnessed at Lanesboro\ Pa., upon 
vour bridge. 

“ ‘ The result of the tests then applied was highly grati¬ 
fying to me, as it was to all present. 

“ ‘ The load sustained was greater than I have ever 
known placed upon a bridge of a single span, and the de¬ 
flections, as will appear by the record reported, were quite 
insignificant. 

“ ‘ I therefore consider your bridge the most perfect 
structure within my knowledge. Its superiority over 












45 

biidges in general use, consists not only in the quality of 
the material and the perfection of the workmanship, but 
in the accurate knowledge displayed in the design, of the 
nature and extent of the various strains to be provided 
against, and the nice disposition of the material for that 
purpose. 

“ ‘ Very respectfully yours, 

‘“S. W. Hall.”’ 

“ Mr. A. J. Centre, Engineer of the Panama Railroad, 
who was one of the committee, was so satisfied with the 
action of the bridge, that he has ordered this plan to be 
built by Mr. McCallum, lor his large structures on the 
Isthmus in spans of over 200 feet. 


The following letters have been kindly tendered for pub¬ 
lication :— 

Office of the New York and Erie Railroad Co. ) 
New York , May 1 5th, 1858. \ 

D. C. McCallum, Esq. 

Dear Sir: 

In reply to yours of the 12th instant, asking an expres¬ 
sion of my opinion in regard to your patent bridges which 
have been built upon this road, I beg leave to state that 
the first two experimental bridges on your plan, one 105 
feet, the other 115 feet in length, were put up late in 
1851, to supply the places of bridges carried off by a Hood, 
and are now standing, in good condition, without having- 
had any repairs, unless it may be a coat of paint; that 
your third bridge, greatly improved, was built in 1851, 
over the Susquehanna River, near Lanesboro’, Pa., with a 
clear span of 190 feet, to supply the place of a Burr span, 
which was shaken to pieces very soon after it was brought 
















46 

into use, and had been for some time supported by “ tres¬ 
tles.” This bridge has now been in use nearly seven years, 
without having cost one cent for watchmen , adjustment, or re¬ 
pairs, except a coat of paint last year, and is now in per¬ 
fect condition. 

Since 1851, thirty-seven others of the larger bridges 
have been rebuilt on your plan; others are now in pro¬ 
gress of reconstruction, and before the close of the year, 
no less than fifty-five of the original and largest bridges 
will have been “ used up,” removed, and yours substituted. 

Besides these, there have been forty-five second track 
bridges built on your plan, making the whole upon this 
road equivalent to one hundred bridges for a single track, 
15,377 feet, or nearly three miles in length. 

To give some idea of the strains to which the bridges on 
this road are subjected, I will remark that the average 
weight of passenger trains, including locomotives, is about 
100 tons, their speed from 25 to 50 miles per hour, and 
their distance run equal to 3,260 times over the road in a 
year. 

The average weight of freight trains last year was 262 
tons; their speed from 15 to 20 miles per hour, and the 
distance run equal to 3,354 times over the road between 
the Hudson River and Lake Erie. 

These do not include the trains employed in ballasting, 
ditching, moving wood, iron, ties, timber, and other sup¬ 
plies for the road. In regard to the durability and economy 
of the bridges built on your plan, I take the liberty to refer 
you to my report to the President on the condition of the 
New York and Erie Railroad, dated December 5th, 1858, a 
copy of which you have herewith, and from which you will 
doubtless be able to glean the information you desire. I 
will, however, embrace this opportunity to again give 



47 * 

my testimony in favor of your bridge over any other in use 
for railroad purposes. 

My opinion is based upon experiments I have made and 
seen made, and upon facts observed through more than 
twenty years of railroad experience; all of which fully 
convince me that the plan adopted by you is not only su¬ 
perior to that of any other in use, but that it possesses 
merits far beyond the advantages for which it has had 
credit, even among its warmest advocates. 

Yours truly, 

S. 8. Po st, 

Chief Engineer. 


New York and Erie Railroad , ) 
Owego , Feb. 9th, 1859. ) 

D. C. McCallum, Esq. 

Dear Sir : 

In answer to yours of the 6th inst., permit me to say 
that I am happy to learn that you are about publishing a 
statement, giving the result of experiments made by you 
several years since, in which models of the various forms 
of bridge trusses in general use were tested. Having aided 
you in making these experiments, I am happy to acknowl¬ 
edge the great benefit I derived therefrom, in determining 
the general action, and what should be the proportions of 
the various parts of any bridge truss. Your bridges upon 
this road are all very permanent and are doing excellent 
service. 

The Susquehanna bridge, of 190 feet clear span, has 
been in active use nearly eight years, and is now, so far as 
I can judge from a recent examination, in perfect condition, 





48 


and has not cost one cent for repairs or adjustment , except that 
it lias had a coat of paint over the joints ; the above will 
apply to all the bridges upon this road built upon your plan. 

As an evidence of the stability of your bridges, I may 
state that the track on the Susquehanna bridge has not 
been “ surfaced ” since the bridge was built, and still re¬ 
mains in good condition. 

Truly yours, 

James Bishop, 

Supt. Bridge Dept. N. Y. and E. R. R. 


Chicago , Feb. 4, 1859. 

D. C. McCallum, Esq. 

Dear Sir: 

I am pleased to learn that you intend issuing a pamph¬ 
let in which you purpose to illustrate your bridge in the 
same interesting manner in which you explained it to me, 
for the benefit of the younger members of the profession. 

I am certain that these illustrations will also prove inter¬ 
esting and useful to all. They will save me the necessity 
of describing the merits of your plan of bridges particu¬ 
larly. 

These bridges were in extensive use upon the New-York 
and Erie Railroad when I was engaged upon that work, 
and I had an excellent opportunity of examining the effect 
of several years use upon them. Your plan, in my opinion, 
combines the most important advantages which are attain¬ 
ed in wooden railroad bridges ; and 1 am able, from my own 
experience, to commend it to the favor of those who have 
such structures to erect. 

Very truly yours, 

Wm. J. McAlpine. 















49 

New Yor/c, Feb. 21st, 18-59. 

D. C. McCallum, Esq., 

Dear Sir : 

Your favor of the 19th inst. is received, inclosing “ proof 
sheets” of a pamphlet you purpose to issue, explaining 
the peculiarities of the “ Inflexible Arched Truss,” and re¬ 
questing my opinion of that plan. 

In answer I would say, that I was engaged as superin¬ 
tending engineer upon the New-York and Erie Railroad 
when the series of experiments referred to in your pamph¬ 
let was authorized by the engineer department, and have 
been familiar with the history of your bridge since then. 

I was also present at the test of the Susquehanna bridge 
in the year 1852, and was, with others, much gratified with 
the result. Being one of the original contractors for the 
construction of the Ohio and Mississippi Railroad, I took 
the liberty of urging the adoption of your bridge upon that 
work, and subsequent experience has given me no cause to 
regret this course. 

I shall at all times take great pleasure in recommending 
your plan of bridge in preference to any other with which 
I am acquainted, trusting that it will receive the high ap¬ 
preciation it so justly merits. 

I remain, yours, &c., 

S. Seymour. 


Middletown, Conn., Feb. 23d, 1859. 

D. C. McCallum, Esq. 

Dear Sir: 

The subject of the proper construction of railroad bridges 
is one of so great importance, that improvements therein 
should be hailed with pleasure; and those instrumental in 

7 













50 

making them should receive the reward due for a valuable 
service rendered. 

Previous to the introduction of railways, a degree of 
perfection had been attained in this country in the construc¬ 
tion of timber bridges of large span, for purposes of com¬ 
mon travel, not excelled, if equalled, elsewhere; witness, 
for example, the bridge over the Hudson, at Waterford, 
and several I might name in Vermont, Pennsylvania, and 
other sections of the country. 

For railway use, however, something more substantial 
and reliable was demanded, especially when the weight of 
locomotives came to be increased from nine to thirty tons, 
and the weight and speed of trains in proportion. 

The lattice plan of Mr. Town, for a time met with favor 
by a portion of my profession, but it was not difficult to 
see that it possessed less merit than a properly framed 
timber bridge. Of the latter, several plans have at differ¬ 
ent times been proposed and patented, all however, in the 
main, involving the same general principle of a truss, com¬ 
posed of an upper and lower line of timbers, either straight 
or curved, connected at intervals by vertical pieces, with 
diagonal braces between. These plans were all more or 
less the result of theoretical views and speculations, and 
although ingenious and possessing many excellent quali¬ 
ties, they wanted that perfect arrangement of parts which 
actual experiment and trial could alone demonstrate to be 
best. 

Your position on the New York and Erie Railroad, in 
charge of bridge constructions and repairs, and subsequently 
as superintendent of that road, afforded you an opportunity, 
which you did not fail to improve, of experimenting on a 
large scale; and it was a source of much gratification to 
me, to be made acquainted, either by yourself or the en- 



51 


gineer in charge, Mr. Post, with the results to which you 
from time to time arrived, and to witness the mode adopted 
by you in forming your trusses, so that the several parts 
should fulfill in the best manner the service required of 
them. I also observed with much interest, the effect upon 
your bridges when in place, produced by the passage over 
them of railway trains, loaded heavily and otherwise, and at 
different rates of speed, and I am free to say, that I know 
of no plan of timber bridge which has stood the test better 
or as well. 

The importance to the public of having our railroads 
provided with the most safe and reliable means of passing 
streams, &c., in localities where structures of earth and 
stone, or iron, are impracticable, from their greater cost or 
otherwise, will at once be conceded ; and in giving my tes¬ 
timony to the merits of your bridge, I do so free from any 
bias of interest in your improvements, save that which 
every citizen should feel in having our railways made as 
perfect as possible, and the risk to life and limb of those 
using them, to the last degree diminished. 

I am gratified to hear that Mr. Post is preparing for pub¬ 
lication a practical treatise on timber bridges, which will 
embody the results of the various experiments made con¬ 
jointly with yourself, and to which I have alluded above. 
I have known Mr. Post for many years, and feel confident 
that he will produce a work of much value to the profes¬ 
sion and the public. His superior judgment and discrim¬ 
ination in mechanical matters, evinced during his profes¬ 
sional career, fully justify this opinion of his treatise in 
advance of its publication. 

Yours very truly, &c., 

Edwin F. Johnson. 















52 

Grand Trunk Railway, 

Engineer Department , 
Sherbrooke Station , 24 1859. 

My Dear Sir: 

I accede, with pleasure, to your request of giving you 
some account of my experience of your bridge, and can 
only say, that I have found it, in all respects, come up to 
the expectations your kind explanation of its principles 
some two years ago, in Montreal, led me to form. 

I have now built six of them, of from 80 to 120 feet 
span, and have three more in hand—one of two spans of 
150 feet each ; another, two of 120 feet; and the third, one 
of 120 feet. These are between Montreal and Island Pond, 
and several more are in course of construction for the line 
between Island Pond and Portland. 

With regard to its merits as a truss, I can imagine 
nothing more perfect, or more admirably adapted for rail¬ 
way traffic. 

The power its peculiar construction affords, for loading 
it down with a w T eight greater than it is ever intended to 
carry, and throwing the amount of this on the counter- 
braces, gives it not only a rigidity, but a life and power 
for resisting and throwing off a load, to which the mere 
tensile and compressive resistance of any beam bridge—no 
matter of what material it may be constructed—can offer 
no comparison. 

I consider that your arched upper chord not only most 
effectively takes the place of an auxiliary arch to a straight 
truss, but goes much further, by (from forming the chord) 
admitting of the structure "being brought into the state of 
rigidity I have mentioned—thereby depriving it entirely of 
the immense amount of vibration hitherto found so deter¬ 
iorating to wooden bridges used for railway purposes— 



53 

and also by the fact of its forming’ part ol and working in 
perfect unison with the truss itself: an advantage which, 
in any other form of the disposal of an arch I have ever 
seen, is unattainable. 

Nothing can be more satisfactory (as I have found it) than 
the action of this bridge under a very heavy load, or more 
in accordance with the evident and simple rules upon which 
it is constructed ; and I have never removed the load after 
the operation of adjusting it lor every day work, without 
having the perfect assurance that I had a bridge which 
would prove, to the fullest extent, efficient and reliable. 

1 would further remark, that not the least of its virtues, 
in my eyes, is the mathematical accuracy with which all 
its proportions can be calculated, thereby giving scope for 
the most true and perfect workmanship, producing not only 
symmetry in appearance, but, by the perfectly correct dis¬ 
tribution of the parts, obtaining from each the full amount 
of strength it was calculated to afford. 

My dear sir, 

Very truly yours, 

D. C. McCallum, Esq. D. Stark. 

New York , March 3d, 1859. 

D. C. McCallum, Esq. 

Dear Sir: 

The bridges on the Eastern Division of the Ohio and 
Mississippi Railroad, constructed under your patent, are 
the only structures of the kind I have had convenient oppor¬ 
tunity of examining. They possessed great strength and 
rigidity, and were well adapted to sustain a heavy railroad 
traffic, and superior to any other wooden bridge tbat lias 
fallen under my observation, Yours very truly, 

Andrew Talcott, Civil Engineer. 










54 


<j Office Ohio Sf Mississippi Railroad Co. 
( Cincinnati , March 2d, 1859. 

D. C. McCallum, Esq. 

Dear Sir: 

All of the important bridges upon the Eastern Division 
of this road, are constructed upon your plan, and it gives 
me pleasure to state, that they have stood the test of heavy 
trains remarkably well. I consider your bridge one ot the 
best, if not the very best, for railroad purposes within my 
knowledge. 

Yours respectfully, 

W. H. Clement, 

Gen’l Sup’t. 


[From the Cincinnati “ Daily Commercial .”) 

“ The test to which the McCallum Railroad Bridge, over 
the Wabash, at Vincennes, was subjected, was more severe 
than indicated by the telegraphic dispatch. We learn from 
Mr. Creveling, of the Ohio and Mississippi Railroad, that 
the steamer Cresent had over eight hundred tons of freight 
in her when she struck the breakwater and swung round in 
a seven mile current, so as to bring her bow with full force 
on the middle span of the bridge. The fact that but two 
chords of the span were injured, and that the bridge was 
not disable -1 lor the passage of trains, is remarkable. 
Another of these bridges sustained the collision of two trains 
meeting in the middle of the span, last summer, and, though 
filled with engines and cars three deep, was not perceptibly 
injured.” 

This bridge consists of four spans of 180 feet each, and a 
draw of 72 feet. 




55 


The foregoing is only intended as an explanation of the 
principles of bridge trusses in general use, in which no 
attempt has been made to compute forces or to furnish the 
methods therefor, the object being to present the subject in 
such form as might induce the student “ to pursue a more 
elaborate investigation of the principles involved in this 
important branch of the professionarid I will take the 
liberty here of earnestly recommending a valuable and 
highly practical work on the Art of Bridge Construction, 
by S. S. Post, Esq., late Chief Engineer of the New York 
and Erie Railroad, which is now in the hands of the pub¬ 
lisher, and shortly to be issued. 

In conclusion, permit me to say that the “ McCallum 
Bridge Company” are prepared to build the “Inflexible 
Arched Truss” in any portion of the United States or the 
Canadas. Any communications, on the subject, addressed 
to the “ McCallum Bridge Company,” No. 110 Broadway, 
New-York; Cincinnati, Ohio ; or to Portland, Maine, will 
receive prompt attention. 

D. C. McCallum, President. 




y 






An edition of the Pamphlet entitled, 

M^CALLUM’S 






EXPLAINED AND ILLUSTRATED, 

BY D. C. McCALLUM, 

Has been printed lor gratuitous circulation; copies 
of which can be had on application to the McCallum 
Bridge Company, at No. 110 Broadway, New-York. 










































































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